This invention relates to rotary die cutting of blanks from thin sheets or webs of material such as paper, paper board, cardboard, plastic film, metal foil, sheet metal and the like. More particularly, this invention relates to improved dies for rotary cutting, die stands for supporting, aligning and driving rotary dies, methods of making rotary dies and methods of rotary die cutting.
One previously known way of rotary cutting is to pass a web of material between a pair of superimposed rotating metal cylinders with one cylinder having a plain cylindrical surface acting as an anvil for cutting elements carried by the other cylinder. The cutting elements projection generally radially outward from the body of the cylinder and have a sharp knife edge with a V shaped cross section. A method and apparatus for producing the V shaped cutting elements as a homogenously integral part of the metal cylinder is disclosed in U.S. Pat. Nos. 3,550,479 and 3,796,851.
U.S. Pat. No. 3,142,233 discloses a way of cutting paper and paper board by passing it between a pair of superimposed rotating dies each of which carries complimentary cutting elements which coact to sever the paper or paper board. The complimentary cutting elements are formed as an integral part of thin and flexible flat metal plates which are wrapped around cylinders and secured thereto by an adhesive or double faced tape. The cutting elements are formed by applying to the plates photocomposed images of the cutting elements in the form of an acid resist and then chemically etching away portions of the remaining surface of the plates so that the cutting elements stand out in shallow relief of about 0.005 to 0.035 of an inch. This chemical etching produces cutting elements having a negative rake.
While these rotary dies are able to cut very thin low density materials such as paper or paper board, they do have a number of significant problems and are unsatisfactory for cutting significantly thicker and higher density materials. In use, it is very difficult to obtain satisfactory registration of the rotating cutting elements so that they will coact to cut the material. Moreover, even when satisfactory registration is initially obtained, in use the thin plates stretch to such an extent that further adjustments must be made while in use and eventually they stretch so much they cannot be registered and hence, produce unsatisfactory cuts and must be scrapped. The plates stretch under the forces created by the cutting action because they must be very thin and soft to enable them to be wrapped around the cylinders.
Furthermore, in use the cutting elements wear or become dull and hence, have a relatively short useful life. They cannot be hardened to increase their useful life because hardened metal plates could not be wrapped around the cylinders. Also, when the cutting elements become dull, due to their shallow relief and negative rake they cannot be resharpened and hence, the metal plates must be discarded. Also because of the negative rake, the cutting elements do not cleanly cut the sheet material, the cut edges are not square, and the cut blanks tend to hang up or catch in the dies and are difficult to strip from the dies.